Disclaimer: the denaturation is the process of protein unfolding from their native state. Since vitamin C is not a protein, it cannot denaturate. I think that the word you're looking for is simply "degradation".
Vitamin C can exist either in its reduced form L-ascorbic acid (L-AA) or in the oxidised form dehydroascorbic acid (DHAA). The active (antioxidant) form of vitamin C is the former; however, there are enzymes in our body that can convert DHAA back into L-AA.
In this article, the authors seek to evaluate the thermal stability of both L-AA and DHAA in broccoli. Their results are the following.
Thermal treatments (for 15 min) of crushed broccoli at 30 to 60 degrees C resulted in conversion of l-AA to DHAA whereas treatments at 70 to 90 degrees C retained vitamin C as l-AA.
So, cooking the vitamin C does not alter its structure and your body will still be able to use it. Another reason to eat broccoli, I guess.